Modified code snippets for multiple top-level packages example
Modified code snippets for multiple top-level packages example

Made them consistent with the snippets given on the Package Discovery
page.
 - Instead of enumerating a list of all the packages in `packages`,
   using `find_packages` or `find:` instead. The `find_packages` call in
   `setup.py` contains a `where` argument. In `setup.cfg`, included the
   section `options.packages.find` with a `where` option.
 - Instead of supplying the same `package_dir` for each package, using
   an empty string to indicate a `package_dir` for all packages.
 - In `pyproject.toml`, using the `where` option instead of
   `package-dir`.
 - Textual changes.

pypa/setuptools,pypa/setuptools,pypa/setuptools